1. Field of the Invention
This invention relates to a process for making a shoe sole, more particularly to a process for making a foamed shoe sole from a foamable polymeric molding composition, such as that containing polyethylene vinyl acetate.
2. Description of the Related Art
Polymeric foams, such as polyethylene vinyl acetate foams, are widely used in making shoe soles. Many processes have been developed in the art in relation to the production of shoe soles from polyethylene vinyl acetate foams. Conventionally, soles are produced from a foamable polyethylene vinyl acetate (EVA) composition via a single-stage forming process, wherein melted EVA composition is introduced into a mold under high temperature and pressure to undergo cross-linking and expansion. For crosslinking and expanding, an EVA composition generally requires a blowing agent, such as azodicarbonamide which not only produces nitrogen and carbon dioxide gases, but also cross-links the EVA polymer. As shown in FIG. 1, a typical process comprises melting an EVA molding material in the form of granules, and injecting the same into a mold at a temperature of 160.degree. C.-175.degree. C. followed by cooling of the molded product for about 4 hours until room temperature (23.degree. C.) is reached. In this process, the temperature at which the molding material is injected should essentially be set at a high temperature of 160.degree. C.-175.degree. C. so as to decompose the blowing agent for cross-linking and expanding. A period of up to 420-540 sec under a pressure of 6.5 kg/cm.sup.2 is required to complete the cross-linking reaction in the mold. After a high pressure and high temperature cross-linking stage, the mold is opened momentarily to allow the molding composition to expand to a greatest volume (about 1.5-2 times the original volume), thereby obtaining a semi-finished product. The temperature at which the semi-finished product is taken out from the mold is about 95.degree. C.-100.degree. C. After cooling to room temperature, the semi-finished product is set to provide a final product.
The above-mentioned process is simple and can be carried out at a low cost, but it suffers from several problems. One of the problems is that the mold for forming the EVA sole has to be free of residual air in order to prevent oxidation of the EVA molding composition in the mold. Such an oxidation will especially occur at the indented parts of the mold as designated by (p) in FIG. 2. Numeral 10 and 11 in FIG. 2 respectively represent the mold and the sole. Upon oxidation, a yellow color that indicates an incomplete cross-linking could result in the sole. In order to expel the residual air, a common practice is to apply vacuum to the mold, or to introduce nitrogen gas into the mold. However, the resulting effect thereof is limited because the mold is not completely sealed.
The presence of air in the mold can also cause the problem of under-filling the mold due to the internal pressure induced by the residual air. The under-filling problem occurs especially at the location where the cross-section of the cavity of the mold has a sudden change, or when the injection rate or pressure is considerably high under poor venting conditions. Under-filling will result in products with insufficient hardness, non-uniform density, and poor skin characteristics. Moreover, as it is difficult to control the amount of the molding material introduced into the mold due to the aforesaid problems, the conventional process usually does not provide products with uniform shape and size. In addition, re-melting or re-molding of defective products for recycle is impossible since the products have been cross-linked.